1. Field of the Invention
The present invention relates generally to wide-range tunable semiconductor lasers and particularly to sampled-grating distributed Bragg reflector (SGDBR) lasers. More particularly, this invention relates to an improved design for sampled grating distributed Bragg reflector (DBR) mirrors.
2. Description of the Related Art
Diode lasers are being used in such applications as optical communications, sensors and computer systems. In such applications, it is very useful to employ lasers that can be easily adjusted to output frequencies across a wide wavelength range. A diode laser which can be operated at selectably variable frequencies covering a wide wavelength range, i.e. a widely tunable laser, is an invaluable tool. Such a “widely-tunable” laser enables real-time provisioning of bandwidth, and a much simplified sparing scheme. By including widely-tunable lasers in a system, if one laser malfunctions, a spare channel purposely left unused, may be configured to the wavelength of the malfunctioning laser and ensure the proper function of the system.
Thus, while diode lasers have provided solutions to many problems in communications, sensors and computer system designs, they have not fulfilled their potential based on the available bandwidth afforded by light-based systems. It is important that the number of channels be accessed and switched between in order for optical systems to be realized for many future applications.
For a variety of applications, it is necessary to have tunable single-frequency diode lasers which can be quickly configured to emit coherent light at any of a wide range of wavelengths. Such applications include sources and local oscillators in coherent lightwave communications systems, sources for other multi-channel lightwave communication systems, and sources for use in frequency modulated sensor systems. Continuous tunability is usually needed over some range of wavelengths. Continuous tuning is important for wavelength locking or stabilization with respect to some other reference, and it is desirable in certain frequency shift keying modulation schemes.
In addition, widely tunable semiconductor lasers, such as a sampled-grating distributed-Bragg-reflector (SGDBR) laser, a grating-coupled sampled-reflector (GCSR) laser, and vertical-cavity surface emitting lasers with micro-electromechanical moveable mirrors (VCSEL-MEMs) generally must compromise their output power in order to achieve a large tuning range. Designs that can provide over 40 nm of tuning range have not been able to provide much more than a couple of milliwatts of power out at the extrema of their tuning spectrum. However, current and future optical fiber communication systems as well as spectroscopic applications require output powers in excess of 10 mW over the full tuning band. Current ITU bands are about 40 nm wide near 1.55 μm and comprise the c-band, s-band and L-band, and it is desired to have a single component that can cover at least one or more of these bands.
Systems that are to operate at higher bit rates may require more than 20 mW over the full ITU bands. Such powers are available from DFB lasers, but these can only be tuned by a couple of nanometers by adjusting their temperature. Thus, it is very desirable to have a source with both wide tuning range (>40 nm) and high power (>10 mW) without a significant increase in fabrication complexity over existing widely tunable designs.
One path to achieving high output power and wide tuning ranges, is to improve upon the conventional sampled grating mirrors or reflectors (which shall be used interchangeably hereinbelow). FIG. 1 shows a typical reflectivity spectrum from a pair of mirrors used within a SG-DBR laser. The design of the SG-DBR is constrained by the desired tuning range, output power and side-mode suppression. It is impossible to simultaneously maximize all three of the above specifications using a SG-DBR, as improving one specification worsens the others. The major concerns when designing a multi-peaked mirror are to achieve the desired coupling constant (κ) and reflectivity (R) for each peak.
The sampled grating approach is limited largely by the fact that the unsampled grating κ is technologically limited by optical scattering to around 300 cm−1. Another limiting factor is that the reflectivity of the multi-peaked mirror falls off at the outer peaks, along with the gain. Therefore, it is desirable to increase the effective κ of each peak as well as compensate for any loss in gain with increased reflectivity. In order to increase the κ of the SG mirror peaks, the sampling duty ratio LB/Λ (the length of sampled portion LB divided by the sampling period Λ) must also increase. This duty ratio, however, is inversely proportional to the wavelength range the multi-peaked SG mirror can effectively cover, which limits the tuning range of a SG-DBR laser. See the mirror reflectivity peak envelope of FIG. 3b. 
Therefore, what is needed in the art is a sampled grating mirror that covers a wide tuning range with the desired κ, as well as having mirror peaks that do not have substantial power dropoffs at the edges of the band.